Dig a hole, place sensor with long enough wire inside, fill back up.
Before doing so, find a sensor that will last longer than the few days, maybe weeks, this one lasts. There are capacitive soil moisture sensors out there (no electrolyses on those), some are even waterproofed (you'd never think that would come in handy when doing soil moisture measurements!).
I had to look up the meaning... never heard of the word... but some methods for measuring evapotranspiration can be found on Wikipedia.